1. Field of the Invention
The present invention relates to the purification of barium salts, and, more especially, to the preparation of high purity barium salts having, in particular, low strontium contents.
The process of the present invention is well adapted, for example, for the production of a purified barium carbonate of such high degree of purity as to be suitable as a precursor of a barium titanate having the same degree of high purity.
2. Description of the Prior Art
The demand in this art for high purity barium salts is ever increasing, in view, for example, of the extensive development of ceramic capacitors based on barium titanate, or of magnetic disks based on barium hexaferrite.
However, barium minerals, whether the sulfate, such as for example in barytine, or the carbonate as in witherite, contain numerous impurities, in particular iron, calcium, sodium and strontium. Other impurities, such as chlorine or sulfur, may be introduced over the course of the conventional processes for the preparation of barium carbonate.
While most of the impurities which usually accompany barium may be eliminated with relative ease by the classical techniques of precipitation/redissolution, the elimination of the trace amounts of strontium remains a serious problem because of the great similarity in the chemical properties of the two elements.
Certain processes are known for the separation of strontium and barium values, based, for example, on chromatography or ion exchange, but these are laboratory separation processes, rather than processes capable of being carried out technically and economically on an industrial scale.
In French Patent No. 86/02,791, assigned to the assignee hereof, an industrial process is described which enables the separation of strontium and barium with a view towards the production of high purity barium salts. The process described is characterized in that an initial aqueous barium solution is intimately contacted with an organic solvent comprising at least one extractant that is essentially insoluble in water, such extractant being selected from among carboxylic acids, monoesters of phosphonic acids and 8-hydroxyquinoline, whereby, after the resulting separation of the phases, a desired final aqueous solution of purified barium values is recovered, as is an organic phase charged with unwanted impurities, in particular strontium.
In the case in which the extractant is an 8-hydroxyquinoline, the '791 patent indicates a preference for 7-(1-methyl-4-ethyloctyl)-8-hydroxyquinoline.
Although this process is capable of providing markedly good purification results as regards the elimination of strontium values, while at the same time requiring relatively few extraction stages (and thus is compatible with industrial level operations), the extent of elimination of the strontium remains inadequate for certain specialized applications.